Cerium-oxygen system

High Temperature X-Ray Diffraction Studies of the Terbium Oxide-Oxygen and Mixed Cerium Terbium Oxide-Oxygen Systems, D.A. Burnham, L. Eyring and J. Kordis, J. Phys. Chem., 72,4424 431 (1968). 

Using these K values calculated for cerium oxides, sulfides, and oxysulfides, Wilson et al. (1974, 1976) have constructed an inclusion precipitation diagram for the Fe-Ce-O-S system (fig. 4), where hct, ho and hs represent the Henrian activities of cerium, oxygen and sulfur, respectively. This composite precipitation diagram gives a clear, qualitative picture of the conditions of phase formations and transformations. 

In addition to platinum and related metals, the principal active component ia the multiflmctioaal systems is cerium oxide. Each catalytic coaverter coataias 50—100 g of finely divided ceria dispersed within the washcoat. Elucidatioa of the detailed behavior of cerium is difficult and compHcated by the presence of other additives, eg, lanthanum oxide, that perform related functions. Ceria acts as a stabilizer for the high surface area alumina, as a promoter of the water gas shift reaction, as an oxygen storage component, and as an enhancer of the NO reduction capability of rhodium. 

In an actual exhaust system controlled by the signal of the oxygen sensor, stoichiometry is never maintained, rather, it cycles periodically rich and lean one to three times per second, ie, one-half of the time there is too much oxygen and one-half of the time there is too Httle. Incorporation of cerium oxide or other oxygen storage components solves this problem. The ceria adsorbs O2 that would otherwise escape during the lean half cycle, and during the rich half cycle the CO reacts with the adsorbed O2 (32,44,59—63). The TWC catalyst effectiveness is dependent on the use of Rh to reduce NO and... 

A washcoat, which provides a high surface area onto which the active catalyst is impregnated. The washcoat typically consists of a mixture of zirconium, cerium and aluminium oxides. Apart from providing high surface area the washcoat also acts as an oxygen storage system (see below). 

In order to improve the resistance of Ni/Al203-based catalysts to sintering and coke formation, some workers have proposed the use of cerium compounds [36]. Ceria, a stable fluorite-type oxide, has been studied for various reactions due to its redox properties [37]. Zhu and Flytzani-Stephanopoulos [38] studied Ni/ceria catalysts for the POX of methane, finding that the presence of ceria, coupled with a high nickel dispersion, allows more stability and resistance to coke deposition. The synergistic effect of the highly dispersed nickel/ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface with oxidation of any carbon species produced from methane dissociation on nickel. 

Berzelius determined the atomic weights of nearly all the elements then known, and was the first chemist to determine them accurately. (19). He referred his atomic weights to oxygen, which, however, he allowed to equal 100, instead of 16 as in our present system. In his little laboratory that looked like a kitchen and in which the sandbath on the stove was never allowed to cool, Berzelius discovered the important elements selenium, silicon, thorium, cerium, and zirconium (18). 

Careful analyses of the thermodynamic data of the praseodymium and terbium oxides led to the construction of their RCL-O2 phase diagrams (Hyde et al., 1966 Hyde and Eyring, 1965). The cerium oxides were studied by means of X-ray powder diffraction (Bevan, 1955) and the CeOx-02 phase diagram was constructed from equilibrium reaction studies at oxygen pressures down to 10-24 atm and temperatures up to 1200 °C (Bevan and Kordis, 1964 Ricken et al., 1984). It is worth to notice that the phase diagrams of Ce0x-02, PrCb-Cb, and Tb0x-02 systems... 

Figure 5 shows the isobar data of CeOx-C>2 system. At one atmosphere with a 150 torr oxygen partial pressure, the cerium oxide almost is dioxide, CeC>2, and there is no phase transition until the temperature increases up 1200 °C. However, when the oxygen partial pressure is about 10-9 torr the oxygen content of the... 

The formation of a rare earth metal oxide on the metal surface, impedes the cathodic reduction of oxygen and thus cathodic inhibition is achieved by the addition of a rare earth metal salt to a system. The surface atom concentration ratio, [Ce/Ce + M], where M is Fe, Al or Zn, is a function of cerium oxide film thickness determined by AES depth profiles as shown in Fig. 12.2. 

Answer 3.6 As with V2O5 in the Contact process for making sulfuric acid, the ability of cerium to adopt transition-metal-like behaviour in switching oxidation states means that the cerium oxide can effectively act as an oxygen-storage system. 

Toward bioapplications Since cerium occurs in both trivalent (+ 3) state and tetravalent (+4) state, it may switch between these two states in a redox reaction in ceria to make excellent oxygen buffers. Therefore, ceria NPs with increased surface area and oxygen vacancies exhibit excellent catalytic activities. In biological systems, reports indicated that ceria NPs... 

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